1. Field of the Invention
The present invention relates to a positioning method for a projection exposure apparatus for exposing mask patterns on a sensitive member coated on a plate such as a semiconductor wafer or a glass plate for a liquid crystal display element and, more particularly, to a projection exposure apparatus having a function of controlling a so-called base line amount of an off-axis alignment system with high precision, and an exposure method.
2. Related Background Art
In a conventional projection exposure apparatus having an off-axis alignment system, as disclosed in Japanese Laid-Open Patent Application Nos. 53-56975 and 56-134737, a fiducial mark plate is fixed on a wafer stage which two-dimensionally moves according to a step-and-repeat scheme while holding a wafer as a sensitive plate, and the distance between the off-axis alignment system and a projection optical system, i.e., a so-called base line amount, is controlled by using this fiducial mark plate.
FIG. 16 shows a main part of a conventional projection exposure apparatus having an off-axis alignment system. Referring to FIG. 16, exposure light from a light source system (not shown) is focused by a main condenser lens 1 to illuminate a reticle R with uniform illuminance. The reticle R is held on a reticle stage 2. The reticle stage 2 holds the reticle R while a center Rc of the reticle R is aligned with an optical axis AX of a projection optical system PL. A pair of reticle marks 3A and 3B for alignment are formed outside a pattern area on the lower surface of the reticle R. Alignment systems 5A and 5B of a TTR (through the reticle) scheme are arranged above the reticle marks 3A and 3B through mirrors 4A and 4B, respectively.
When an exposure is to be performed, a pattern of the reticle R is projected/exposed on each shot area of a wafer W on a wafer stage 6 through the projection optical system PL. A wafer mark for alignment is formed in each shot area of the wafer W. A fiducial mark plate 7 having a mark 8 for alignment formed thereon is fixed on a portion near the wafer W on the wafer stage 6. When the wafer stage 6 is positioned such that the fiducial mark plate 7 is set at a position almost conjugate to the reticle marks 3A and 3B within the projection field of the projection optical system PL, the reticle mark 3A or 3B and the mark 8 are simultaneously detected by the alignment systems 5A and 5B above the reticle R. A distance La between the reticle mark 3A (or 3B) and the center Rc of the reticle R is a design value, and the distance between the projection point of the reticle mark 3A and the optical axis AX on an image plane of the projection optical system PL (the surface of the fiducial mark plate 7) is represented by La/M, where M is the magnification of the projection optical system PL when viewed from the wafer W side to the reticle R side. If the projection optical system PL is a 1/5 reduction projection optical system, M=5.
An off-axis wafer alignment system 9 is arranged outside the projection optical system PL. The optical axis of the wafer alignment system 9 is parallel to the optical axis AX of the projection optical system PL. An index plate 10 having an index mark formed thereon is fixed inside the wafer alignment system 9. The surface of the index plate 10 on which the index mark is formed is conjugate to the surface of the mark 8.
For example, a base line amount BL of the wafer alignment system 9 is defined as the distance between the optical axis, of the wafer alignment system 9, located on the wafer stage 6 and a projection point, of the center Rc of the reticle R, formed by the projection optical system PL. When the base line amount BL is to be measured, the wafer stage 6 is driven to move, for example, the mark 8 of the fiducial mark plate 7 to a position A immediately below the wafer alignment system 9. The positional offset amount of an image of the mark 8 with respect to an index mark in the wafer alignment system 9 is read together with the coordinates of the wafer stage 6 at this time. The coordinates of the wafer stage 6 are measured by a laser interferometer with high resolution. With this operation, the coordinates (X1,Y1) of the wafer stage 6 when the mark 8 is on the optical axis of the wafer alignment system 9 can be obtained.
Subsequently, the wafer stage 6 is driven to sequentially move the mark 8 of the fiducial mark plate 7 to positions near positions C and B which are conjugate to the reticle marks 3A and 3B. The positional offset amounts of an image of the mark 8 with respect to the reticle marks 3A and 3B are read together with the corresponding coordinates of the wafer stage 6. With this operation, the coordinates (X2,Y2) of the wafer stage 6 when the mark 8 is located at the middle point between the reticle marks 3A and 3B, i.e., a conjugate point of the center Rc of the reticle R, can be obtained. Therefore, the base line amount BL can be obtained as the distance between the coordinates (X1,Y1) and the coordinates (X2,Y2). This base line amount BL is used afterward as a fiducial amount used for positioning of each shot area of the wafer W within the exposure area of the projection optical system PL on the basis of the coordinates of the wafer mark on the wafer W which are read by the wafer alignment system 9.
Letting XP be the distance between the center of a given shot area on the wafer W and a corresponding wafer mark in the X direction, X3 be the position of the wafer stage 6 in the X direction when the wafer mark is aligned with the optical axis of the wafer alignment system 9, and BLx be the X-direction component of the base line BL, the wafer stage 6 may be moved by the amount given by the following expression in order to align the center of the shot area designated by the wafer mark with the projection point of the center Rc of the reticle R: EQU X3-BLx-XP
The moving amount in the Y direction can be represented by a similar expression. Note that this expression is based on the arrangement shown in FIG. 16, and different calculation methods are used depending on the positions of the reticle marks 3A and 3B or the arrangement of the wafer alignment system 9.
With any expression for calculation, after the position of each wafer mark on the wafer W is detected in advance by using the off-axis wafer alignment system 9, each shot area on the wafer W is positioned and exposed within the exposure area of the projection optical system PL in accordance with each detected position, thereby accurately superimposing and exposing a pattern of the reticle R on each shot area of the wafer W.
Since the superposition precision deteriorates even though the reticle R is mounted such that it is rotated about the optical axis of the projection optical system PL from a fiducial angle, the rotational amount (reticle rotation) of the reticle R is measured as follows. Referring to FIG. 16, the wafer stage 6 is driven to sequentially move the mark 8 of the fiducial mark plate 7 to the positions B and C, and the relative positional relationships between a conjugate image of the mark 8 and the reticle marks 3A and 3B are measured by the alignment systems 5A and 5B, respectively. With this operation, the rotational amount of the reticle R based on the traveling direction of the wafer stage 6 is measured. In the conventional apparatus, the rotational amount of the reticle R is adjusted by a reticle fine movement mechanism (not shown) such that the rotational amount of the reticle R based on the traveling direction of the wafer stage 6 is set to be a predetermined allowable value or less.
In the conventional projection exposure apparatus, since the base line amount of the off-axis wafer alignment system 9 and the rotational amount of the reticle R are measured on the basis of the traveling direction of the wafer stage 6, these amounts cannot be measured with high precision because of a measurement error or the like in a coordinate measurement system for the wafer stage 6. More specifically, an error is included in the measurement value of the coordinates of the wafer stage 6 owing to the influences of fluctuations in the optical path of a laser interferometer for monitoring the coordinates of the wafer stage 6, an error in the reset position for the initial coordinates of the wafer stage 6, an error in the set position of a movable mirror for reflecting a laser beam above the wafer stage 6, and the like. Therefore, errors are included in the base line amount and the rotational amount of the reticle R based on the measurement value.
In the conventional apparatus, the base line amount of the wafer alignment system 9 is obtained every time the reticle R is replaced.
In the conventional projection exposure apparatus, the base line amount is measured every time the reticle R is replaced, and exposure is performed after, for example, several hundred wafers are aligned by using the wafer alignment system 9. Since exposure is continuously performed, the temperature near the wafer stage 6 changes. For this reason, a drift occurs in the relative distance between the detection center of the wafer alignment system 9 and the projection position of the center of the reticle R. As a result, the superposition precision of a pattern image of the reticle R and a pattern formed on the wafer deteriorates.
FIG. 15A shows an example of a change in the drift amount (unit: .mu.m) of the base line amount. In FIG. 15A, the abscissa represents the number of wafers to be exposed after the reticle R is replaced, and the ordinate represents the drift amount of the base line amount. As is apparent from FIG. 15A, even if measurement of a base line amount is performed and the drift amount is set to be 0 at time Q after replacement of the reticle R, the drift amount gradually increases.
In order to eliminate this inconvenience, U.S. Pat. No. 4,897,553 discloses a method of measuring a base line amount every time a wafer is replaced, and a method of measuring a base line amount every time a predetermined number of wafers (e.g., several wafers) are exposed. If, however, a base line amount is measured every time a wafer is replaced, the throughput is reduced because measurement of a base line amount takes much time. When a base line amount is to be measured every time a predetermined number of wafers are replaced, measurement of a base line amount is performed at almost equal time intervals, i.e., times R1, R2, R3, . . . , as shown in FIG. 15B. However, the drift amount of the base line amount does not change linearly with-respect to the number of wafers to be exposed (substantially proportional to time). If, therefore, a wafer alignment system in which the drift amount increases immediately after an exposure operation is started is used, the drift amount may exceed an allowable value.